The invention relates to a tunable phase plate for charged particle imaging systems.
When an object is imaged using a beam of charged particles, particularly electrons, features in the object may provide contrast by influencing the intensity of the transmitted, reflected or scattered beam. Some features in the object do not influence the intensity but rather the phase of the beam. To transform the phase variation into an intensity variation, i.e. into useful image contrast, a phase plate can be used to introduce a phase difference between the beam that has interacted with the object and that which has not interacted.
Especially for transmission electron microscopy, various kinds of phase plates have been developed over the last decades. (K. Nagayama, “Another 60 years in electron microscopy: development of phase-plate electron microscopy and biological applications”, Journal of Electron Microscopy 60 (Supplement 1), S43-S62 (2001)) gives a review. Most of those phase plates are literally plates of some non-conducting material that the beam has to pass; the phase change is brought about by the refractive index of this material, which changes the optical path length. The downside to such plates is that a charge accumulates on them, which influences the beam and de-grades the image.
(G. Balossier, N. Bonnet, “Use of an electrostatic phase plate in TEM. Transmission electron microscopy: Improvement of phase and topographical contrast”, Optik 58, 361-376 (1981)) discloses a phase plate made of a very thin wire that is mounted perpendicular to the electron beam. Where the beam directly impinges on this wire, secondary electrons escape, creating a localized net positive charge that shifts the phase of the beam. Unfortunately, as pointed out in the review article (R. M. Glaeser, “Methods for imaging weak-phase objects in electron microscopy”, Review of Scientific Instruments 84, 111101 (2013)), the amount of phase shift that such a configuration (and similar ones based on self-charging under the beam) produces cannot be understood with any available physical or mathematical model yet. Therefore, in most situations the performance of this kind of device will be detrimental rather than favourable on the image.